Molecular conjugates are formed by covalently bonding two or more molecules to each other. Such conjugates may exhibit useful properties that combine, exceed or differ from the properties of the components. For example, our U.S. patent application Ser. No. 002,368 now U.S. Pat. No. 4,701,521 ("Method of Effecting Cellular Uptake of Molecules," filed on Jan. 10, 1979) describes a method of bonding anti-cancer drugs which enter cells at low rates to cationic polymers which enter cells at relatively higher rates. Such drugs enter cells at higher rates when administered in conjugate form than when administered as unmodified drug.
Certain molecular conjugates, such as the conjugate mentioned above, may be referred to as macromolecular drug carriers. The components of such conjugates may be regarded as carriers or passengers, depending upon their primary function. The primary purpose of a carrier in the context of this invention is to increase the transport and delivery of a passenger molecule to a desired location. Once the carrier reaches a desired location, it may perform other functions. Passenger molecules may comprise drugs, antibodies, antigens, lectins, dyes, stains, tracers, or other substances that perform at least one useful function upon reaching a desired location with the aid of a carrier molecule.
According to current concepts of cell biology, ingested macromolecular carriers are transported to lysosomes where they are subjected to the action of lysosomal enzymes [1]. If a carrier is a proper substrate for one or more of these enzymes, it normally is hydrolyzed and is digested into diffusible metabolites. Such metabolites are normally excreted or reutilized.
In addition to containing hydrolytic enzymes, lysosomes tend to be substantially more acidic than other compartments or fluids within a cell or body. For example, the pH of blood is about 7.3 to 7.4 [2]. However, the pH of a lysosome is about 4.8 [3], and it has been suggested that the pH within a lysosome can be as low as 3.8 during the initial stage of digestion [4].
Conjugation of a drug to a carrier molecule may reduce the desired activity of the drug until the carrier is hydrolyzed or digested. There are at least two mechanisms by which this may occur. First, conjugation may alter the size or shape of a drug, thereby impeding steric interaction with a target molecule. For example, the anti-cancer drug methotrexate kills cells by binding to and inactivating an essential enzyme, dihydrofolate reductase (DHFR). Conjugation of methotrexate to poly(lysine) causes steric hindrance which substantially reduces the ability of methotrexate to inactivate DHFR. However, when the poly(lysine) is digested or hydrolyzed, the methotrexate recovers its activity. Second, conjugation may alter a functional group that is essential for pharmacological activity. For example, in certain drugs like Daunomycin, the chemical group that normally is most suitable for conjugation is a functional amine group. When Daunomycin is linked to another molecule through that group, the resulting conjugate is much less pharmacologically active. The Daunomycin recovers its full activity only if it is released from the conjugate in unmodified form.
A drug-carrier conjugate that relies upon enzymatic processes to break the conjugate into its components, thereby reactivating the drug, is subject to certain limitations, including the following. First, certain carriers and conjugates may not be susceptible to lysosomal hydrolysis. Second, enzymatic action may release a modified, less active form of the passenger drug. Third, certain carriers and drugs may inhibit or modify the normal enzymatic functions of lysosomes.
For these and other reasons, it would be useful to achieve the cleavage of drug-carrier linkages by non-enzymatic techniques. This can be achieved by taking advantage of the acidic pH of lysosomes by creating molecular conjugates that are sensitive to mild acidity, such as exists in the interior of a lysosome.